In the past, the separation of C.sub.4 hydrocarbons from a gaseous mixture containing hydrogen and perhaps also lighter hydrocarbons has been performed by partial condensation. The feed gas is cooled to a temperature at which the residual hydrocarbon content in the vapor satisfies the requirement of hydrogen purity for the use to which the uncondensed vapor phase is to be put. The liquid phase is then recovered as liquid product. This liquid is normally warmed to ambient temperature to recover its refrigeration.
The Oleflex process produces a gaseous effluent; but this is then cooled and condensed to a liquid fraction containing substantially all the C.sub.4 hydrocarbons, which are then separated from each other, the isobutene being a product stream and the liquid isobutane being recycled. The liquid fraction will be about 40-50% of the effluent; and this requires an important supply of refrigeration. As in most cryogenic processes, this refrigeration can be supplied either by a liquid refrigerant, mechanical refrigeration or expansion engines, or a combination of these.
In the past, the liquid isobutane feed stock has been cooled to about -80.degree. F. and has been vaporized by mixing it with hydrogen recycled at lower pressure to produce refrigeration. The feed gas is partially condensed by the refrigeration from the vaporization of the liquid feed stock. The feed gas is further cooled by the use of expansion engines to provide further product recovery. Ordinarily, two expansion engines are used in series, to maximize the efficiency of the equipment, since the final expanded gases are quite light and have an average molecular weight of about 3 to 4.
Such a prior art method and apparatus is shown in FIG. 1 of the accompanying drawings. In FIG. 1, the feed gas, which is the effluent from an Oleflex reactor (not shown), enters the warm end of exchanger 1 via conduit 3 and is cooled and partially condensed and phase separated in separator 5. The liquid from separator 5 is expanded isenthalpically through valve 7 and further separated in separator 9, the liquid from separator 9 being increased in pressure via pump 11 sufficiently that when returned via conduit 13 through exchanger 1, the product leaves exchanger 1 in liquid phase at 15 and then the isobutene is converted to methyl tertiary butyl ether in a conventional unit (not shown), whereafter the latter is separated by conventional fractionation from the isobutane, the liquid isobutane being recycled to the liquid feed 17.
From separator 5, the vapor is expanded isentropically in expansion engine 19 and fed to a phase separator 21. The liquid separated in 21 is expanded isenthalpically through valve 23 to the same pressure as the liquid expanded in valve 7 and joins the latter in passing to separator 9.
The vapor separated in 21 is divided, a portion being removed through conduit 25 as hydrogen product after being used to cool exchanger 1, and the remainder being further expanded isentropically in an expansion engine 27, whose output is phase separated at 29. The liquid from separator 29 is expanded isenthalpically through valve 31 to the same pressure as prevails downstream of valves 7 and 23 and is similarly sent to separator 9.
The vapor from separator 29 is removed through conduit 33 and directed through exchanger 1 to cool the exchanger, whence a portion can be removed, if desired, as hydrogen product at 35; but in any event a large portion of hydrogen from conduit 33 will be merged with liquid feed 17, which has been expanded in valve 37 isenthalpically to the same pressure as in conduit 33, and the mixed liquid and vapor thus produced is completely vaporized in passage toward the warm end of exchanger 1 and leaves exchanger 1 via conduit 39 as the gaseous feed to the Oleflex reactor.
Of course the liquid isobutane in conduit 15 is not sufficient to constitute the liquid feed 17; and so a separate source of liquid isobutane (not shown) provides the required makeup liquid isobutane.
But the process described above has several drawbacks, as follows:
1. The pressure of the feed gas entering via conduit 3 must be high, in order to produce sufficient refrigeration in the expansion engines 19 and 27. Such a high feed gas pressure is not desirable, because the compression equipment is expensive and power consumption is high.
2. An equally unsatisfactory alternative to high feed gas pressure, is the use of mechanical refrigeration at a low temperature level of about -80.degree. F. This can be provided e.g. by an ethylene refrigeration unit in cascade with a propane unit. But such refrigeration equipment is, like the compression equipment, very expensive and its power consumption is high.
3. The purity of the hydrogen in conduit 33, which will be recycled with the liquid feed via conduit 39 to the Oleflex reactor, is low. This results in lower unit capacity and higher operating cost, due to the undesirable presence of lighter hydrocarbons such as methane in this stream.